Benzene from aromatic mixtures

Broughton, D., (1981) Production of pure M-xylene and pure ethyl benzene from a mixture of C8 aromatic isomers. US Patent 4,306,107. 

Aromatic Isomer Separation. Recent activity directed to producing pine aromatic hydrocarbons has been concerned primarily with separating isomers from aromatic mixtures. The problem does not arise with benzene and toluene, but is encountered first with Cg aromatic mixtures some of these isomers have been separated commercially since World War II to provide intermediates for chemical synthesis. 

Although a lot of effort has been spent in the development of membranes for the separation of mixtures of nonpolar organic components no large-scale application has yet been reached. Of specific interest is the separation of olefins from paraffins, e.g. propene from propane, aromatics like benzene or toluene from aliphatic hydrocarbons or the separation of the xylene isomers. A number of different membranes are reported in the patent literature [27]. The first pilot plants are being operated and results reported for the separation of sulfur-containing aromatics from gasoline [28], or for the separation of benzene from a mixture of saturated hydrocarbons [29]. 

The most important benzene aromatics in terms of quantity are benzene and p-xylene. Since the production from reformer gasoline, pyrolysis gasoline and coke-oven benzole is frequently inadequate to meet the demand, isomerization, disproportionation and dealkylation methods have been developed to complement the direct production from aromatics mixtures. 

Arosolvan process A process for the extraction of benzene and toluene from a mixture of aromatic and saturated hydrocarbons using a mixture of water and N-methylpyrrolidone. The process is used when naphtha is cracked to produce alkenes. To prevent extraction of alkenes these are saturated by hydrogenation prior to extraction. 

Manufacture The xylenes are obtained with benzene (and toluene) from the catalytic reforming of naphtha and separated from the aromatic mixture by distillation. From the mixed isomers, the ortho- can be obtained by distillation because its boiling point is sufficiently different. The meta- and para- are separated by either selective adsorption or by crystallization. 

Poro-xylene is an industrially important petrochemical. It is the precursor chemical for polyester and polyethylene terephthalate. It usually is found in mixtures containing all three isomers of xylene (ortho-, meta-, para-) as well as ethylbenzene. The isomers are very difficult to separate from each other by conventional distillation because the boiling points are very close. Certain zeoHtes or mol sieves can be used to preferentially adsorb one isomer from a mixture. Suitable desorbents exist which have boiling points much higher or lower than the xylene and displace the adsorbed species. The boihng point difference then allows easy recovery of the xylene isomer from the desorbent by distillation. Because of the basic electronic structure of the benzene ring, adsorptive separations can be used to separate the isomers of famihes of substituted aromatics as weU as substituted naphthalenes. 

Isopropenylbenzofuran (124, Scheme 30) affords good yields of the adducts 123 and 125 on separate reaction with maleic anhydride and tetracyanoethylene. With but-3-en-2-one, 2-isopropenylbenzofuran (124, Scheme 31) affords the adducts 126 and 127 in a combined yield of 29%. When the crude product was dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in boiling benzene, the aromatized product 128 (6%) was obtained. It was accompanied by the dicyanodibenzofuran 129, which was found to arise from the excess diene present in the reaction mixture. A speculative mechanism is shown. 

From the relative roles of the primary and tertiary radicals which are statistically formed on p-cymene, the termination constant of p-cymene should lie between that of cumene and that of toluene or xylene. Experimentally, this interpretation can be easily checked (12) by comparing the oxidation of p-cymene diluted by benzene with aromatic hydrocarbon mixtures having equal concentrations of methyl and isopropyl groups in comparable mixtures (Table V). 

Enantiopure epoxides and vicinal diols are important versatile chiral building blocks for pharmaceuticals (Hanson, 1991). Their preparation has much in common and they may also be converted into one another. These chirons may be obtained both by asymmetric synthesis and resolution of racemic mixtures. When planning a synthetic strategy both enzymic and non-enzymic methods have to be taken into account. In recent years there has been considerable advance in non-enzymic methods as mentioned in part 2.1.1. Formation of epoxides and vicinal diols from aromatics is important for the break down of benzene compounds in nature (See part 2.6.5). 

In the field of aromatic separation, the trend of research is toward isolation of pure compounds for chemical purposes. Benzene, toluene, and some of the C8 aromatics have been separated and used commercially. However, the physical properties of the C9 and Cio aromatic hydrocarbons found in reformed stocks show that other aromatics could be separated from these mixtures by distillation, crystallization, or extraction processes. It is reasonably certain that if sufficient demand develops for the pure compounds, processes for their separation will become available. Present information indicates that perhaps methylethylbenzenes and trimethylbenzenes could be isolated in relatively high purity by distillation from aromatic stocks obtained by hydroforming, but no information is available as to their industrial uses. Similarly, durene (1,2,4,5-tetramethylbenzene) possibly could be isolated from its homologs by crystallization. Furthermore, large... 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Application The Sulfolane process recovers high-purity aromatics from hydrocarbon mixtures by extractive distillation (ED) with liquid-liquid extraction or with extractive distillation (ED). Typically, if just benzene or toluene is the desired product, then ED without liquid-liquid extraction is the more suitable option. 

A zeotropic and extractive distillations have been used through the years in the chemical industry to separate mixtures where the relative volatility of the key components is very close, or equal, to unity. Applications from the classical dehydration of alcohol with benzene (1) to more recent ones such as the propylene-propane separation (2) and aromatics recovery from hydrocarbon mixtures with N-methylpyrrolidone (3), indicate a continuous interest through the years in this area. 

The reaction of /3-di ketones with metal alkoxides is generally carried out in anhydrous aromatic solvents, affording the desired metal diketonate and delivering alcohol (Equation (19)) that can be removed from the mixture reaction by fractional distillation of the alcohol-benzene... 

Lhomme and Ourisson,11 in working on the oxidation of camphanols with lead tetraacetate in benzene found that there was considerable risk that very volatile products would be entrained during removal of the benzene by distillation. Since sulfolane has been used in the industry for the extraction of aromatics from hydrocarbon mixtures, they tried adding pentane and then extracting the solution several times with the nonmiscible sulfolane. The residual pentane solution could then be washed with water for the removal of sulfolane and recovery of the volatile cyclic ethers formed in the oxidation. The procedure was verified by showing that more than 70% of camphene could be recovered in this way. 

An index whose value is high, suggests that the stationary phase strongly retains the compounds that contain the corresponding organic functions. This leads to an improved selectivity for this type of compound. In the same way, to separate an aromatic hydrocarbon from a mixture of ketones, a stationary phase would be selected for which the McReynolds constant for benzene is rather different to that of pentanone. These differences in retention indexes are provide by suppliers. 

The mechanism of the catalytic hydroxylation of aromatic hydrocarbons by hydrogen peroxide has been reviewed (ref.8). The hydroxylation of benzene or toluene by peroxydiphosphate (ref.9) and peroxydisulphate, (more comnronly termed persulphate) (ref. 10) in aqueous (0.05-1.OM) acid in the presence of Cu(ll) has been found to be similar. Phenol (15.6%), diphenyl (5.2%), and 2-and 4-nitrophenols (11.7% and 5.6% respectively) resulted from a mixture of benzene and nitrobenzene in water at 80 C with peroxydisulphate. 

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